Continuity of Exterior Bearing Wall Fire Rating at Building Stepback

[Harrison Staab]

Where a building steps back, does the supporting walls/structure need to maintain the same fire rating of the exterior bearing wall above?

Specific application:
California Building Code 2019 (sim. to IBC 2018)
Six stories, 5 IIIA over 1 IA
Group R-2 Multi-family apartment
Type IIIA construction with 2-hr exterior bearing walls
At fourth floor, the building steps back, with a 2-hr exterior bearing wall directly supported by a 1-hr fire partition (a dwelling unit separation wall).
Does this supporting fire partition wall need to be 2-hr rated?



I believe section 705.6 is at play:

705.6 Structural Stability

Exterior walls shall extend to the height required by Section 705.11. Interior structural elements that brace the exterior wall but that are not located within the plane of the exterior wall shall have the minimum fire-resistance rating required in Table 601 for that structural element. Structural elements that brace the exterior wall but are located outside of the exterior wall or within the plane of the exterior wall shall have the minimum fire-resistance rating required in Tables 601 and 602 for the exterior wall.

Would the wall below the exterior wall be considered "within the plane of the exterior wall" and thus be required to have a 2-hr fire rating as required by Table 601 for exterior bearing walls?

 

[Harrison Staab]

And bonus follow-up question. What if the supporting element below is a beam. Is this a 1-hr rated primary frame? Is this 2-hr to match the wall above?

 

[Yikes]

Harrison, I was doing a search on 510.5 and came across your question that appears to have gone unanswered. I'm going to say in my own opinion, the answer is "no". The interior support wall in your illustration is not "within the plane" of the exterior wall. It is "below the plane" of the exterior wall.

I believe the intent of the phase "in the plane" is to address situations such as occur in typical platform framing, where someone builds 2-hour rated walls, but between the sill plates the floor structure is just a single rim joist/blocking that's only good for one hour.
See https://www.woodworks.org/cad-revit-details/light-frame/type-iiia-exterior-wall-floor-intersection/

 

[Paul Sweet]

IBC 704.1 says "... The fire-resistance ratings shall be not less than the ratings required for the fire-resistance-rated assemblies supported by the structural members."

 

[Yikes]

Paul, you are correct, IBC 704.1 appears to be more stringent and therefore supersedes my previous interpretation of 705.6.